THE SKELETON 4I 



used, one of which should be bisected sagittally, a second 

 should have the roof of the cranial and nasal cavities removed, 

 and the third should be cut transversely on a line joining the 

 middle of the orbits. The ethmoid is usually visible externally 

 as a small rhomboid plate on the medial wall of the orbit 

 between the frontal, lachrymal, and palatine bones. It con- 

 sists of four parts: the paired clhmoturhinals or lateral ethmoids 

 (Fig. 19), the mesethmoid, and the cribriform plate. The eth- 

 moturbinals are in the form of scroll-like lamina} which project 

 forward from the transverse cribriform plate into the nasal 

 cavities. The portion which appears externally in the medial 

 wall of the orbit is the os planum. In the recent state, the 

 surfaces of these bones within the nasal cavity are covered 

 with a mucous membrane over which the first pair of cranial 

 nerves (olfactory) are distributed. The delicacy of the sense 

 of smell is proportional to the development of the ethmotur- 

 binals. In most orders of animals live scrolls are present, 

 but in Echidna there are six and in some Ungulates there are 

 eight, while in adult Primates there are only from one to three 

 more, however, being present in the embryo. 



The mesethmoid is the perpendicular plate of bone which, 

 prolonged craniad by cartilage, separates the nasal cavity 

 into two portions. Caudad it is united to the cribriform 

 plate, dorsally it articulates with the median descending plates 

 of the frontals and the nasals, and ventrally it articulates with 

 the vomer and presphenoid. The cribriform plate is the cau- 

 dal portion of the ethmoid (Fig. 19), which extending trans- 

 versely between the frontals, separates the cranial cavity from 

 the nasal cavity. It is pierced by many pinhole foramina for 

 the exit of the olfactory nerve. In Ornithorhynchus (duck-bill 

 of Australia) there is a single large foramen in the cribriform 

 plate, as is also the case in birds. 



The temporal is a paired bone lying at the base and side 

 of the skull. It contains the organs of hearing. It consists 

 of four parts: the squamous or expanded portion (Fig. 17), 



